Recently, much attention has been paid to magnetic recording media having thin magnetic metal films produced by spattering or vacuum deposition techniques as magnetic layers. The main reason why those thin magnetic metal films are paid much attention as magnetic recording media is, as well known in the art, that they can realize higher recording densities compared with prior materials having coated magnetic powder. It has been known that Co-containing alloys can exhibit good magnetic coercive force and squareness ratio as magnetic materials for such thin magnetic metal films. In particular, CoPt alloy thin layers have recently been paid much attention for industrial purpose as materials capable of realizing higher recording density because they show high magnetic coercive force and residual flux density.
Further, in magnetic recording media for magnetoresistive (MR) type heads, a lower medium noise level is required rather than an output magnitude. As media meeting to this demand, there have been known a magnetic recording medium comprising a substrate, two magnetic layers of CoPtCr provided on the substrate and a nonmagnetic spacer layer comprising Cr provided between the magnetic layers (Japanese Patent Un-examined Publication KOKAI! No. 2-210614). Because the magnetic layer of this magnetic recording medium is splitted into two layers by the non-magnetic spacer layer, it has a smaller thickness of each magnetic layer compared with that of those media comprising one magnetic layer and having the same total magnetic layer thickness and hence it can reduce noises upon reproducing recorded signals.
Magnetic recording media having the structure described above can indeed reduce noises because of the smaller thickness of each magnetic layer obtained by splitting the magnetic layer with the non-magnetic spacer layer. However, various evaluations of these magnetic recording media in terms of properties required for practical use have revealed that they have drawbacks such as follows:
(1) Reduction of magnetic coercive force of these media is more significant compared with those having one magnetic layer. Therefore, even if Pt alloy magnetic layers having more excellent magnetic coercive force compared with other materials are used as magnetic layers, they cannot make the most of their advantages. PA0 (2) Their overwriting properties are insufficient.
Therefore, the first object of the present invention is to provide magnetic recording media comprising two or more CoPt alloy magnetic layers separated by one or more non-magnetic spacer layers and showing low medium noise and excellent magnetic coercive force as well as excellent overwriting characteristics and methods for producing them.
Further, the prior media have a drawback that they show poor corrosion resistance and therefore reproducing errors may be caused by corrosion generated during their use for a long period of time.
Therefore, the second object of the present invention is to provide magnetic recording media which satisfy the first object of the present invention described above and, in addition, are excellent in corrosion resistance.
Further, the third object of the present invention is to provide magnetic recorders showing a high S/N ratio upon signal recording and have a high recording capacity and excellent resistance to environmental variance.
Meanwhile, underlayers of Cr in magnetic recording media comprising magnetic layers of CoNiCr alloys, CoCrTa alloys or the like have been known to be able to provide high magnetic coercive force (see, for example, IEEE TRANSACTION ON MAGNETICS VOL. MAG-3, NO. 3 (1967), p. 205-507).
However, when CoPt alloy magnetic layers are used, underlayers composed of a sole component of Cr disadvantageously deteriorate C-axis orientation. Lattice constants of CoPt alloy magnetic layers are larger than crystalline lattice constants of magnetic layers composed of CoNiCr alloys, CoCrTa alloys and the like because of the larger atomic radius of Pt. Therefore, their conformity with the underlayers composed of a sole component of Cr in terms of atomic arrangement is deteriorated and as a result the C-axis orientation is deteriorated.
As a means for solving this problem, it has been proposed that a second metal (dissimilar metal) is added to the Cr underlayers to make crystalline lattice constants larger. By utilizing Cr alloy (such as CrV) underlayers where a dissimilar metal is added to change lattice constant of the alloy underlayers, the C-axis orientation at layer surfaces can be improved to enhance magnetic coercive force and squareness ratio (Japanese Patent Publication KOKOKU! No. 4-16848).
However, the present inventors study revealed that medium noise is sharply increased by the addition of a dissimilar metal to the Cr underlayers.
Therefore, the forth object of the present invention is to provide magnetic recording media comprising a Cr alloy underlayer and two or more CoPt alloy magnetic layers separated by one or more non-magnetic spacer layers and showing low medium noise and methods for producing them.
In addition, the fifth object of the present invention is to provide magnetic recording media comprising a Cr alloy underlayers and CoPt alloy magnetic layers and showing high magnetic coercive force, squareness ratio and low medium noise, and methods for producing them.
Furthermore, the sixth object of the present invention is to provide magnetic recording media comprising a Cr alloy underlayer and two or more CoPt alloy magnetic layers separated by one or more and non-magnetic spacer layers and showing low medium noise, excellent magnetic coercive force and overwriting properties, and methods for producing them.
As a result of the present inventors' research, it was found that, in magnetic recording media comprising two or more CoPt alloy magnetic layers separated by one or more non-magnetic spacer layers, magnetic coercive force and overwriting characteristics are deteriorated depending on compositions of materials of the layers separating the magnetic layers and their production conditions. Further, it was found that, in such magnetic recording media as described above having CoPt alloys as the magnetic layers, medium noise can be reduced while magnetic coercive force and overwriting properties are maintained by utilizing alloys consisting mainly of Cr and Mo for the non-magnetic spacer layers separating the magnetic layers and adjusting crystalline lattice spacings of (002) faces in the CoPt alloy magnetic layers to crystalline lattice spacings of (110) faces in the Cr alloy non-magnetic underlayers immediately under the magnetic layers.
Furthermore, also as a result of the present inventors' research, it was found that ununiformity of crystalline particle size and crystallinity of the Cr underlayers are reduced by adding dissimilar metals such as Mo to the layers through transmission electron microscopy. That is, it was found that, because growth of the CoPt alloy magnetic layers such as CoPtCr to be laminated on the Cr alloy underlayers is strongly influenced by particle sizes and crystallinity of the underlayers, ununiform particle sizes and bad crystallinity of the Cr underlayers lead to ununiform crystalline particle sizes and markedly bad crystallinity of the magnetic layers and they increase medium noise.
Based on the above, a second underlayer comprising a Cr alloy obtained by adding a dissimilar metal to Cr was experimentally laminated on a layer (underlayer) having uniform crystalline particle size and good crystallinity and, as a result, it was observed that uniform crystalline particle size and good crystallinity of the Cr alloy underlayer were obtained. However, using two underlayers as described above still could not reduce medium noise sufficiently.
Therefore, further research was conducted and it was found that medium noise can be markedly reduced by adjusting crystalline lattice spacings of (002) faces in the CoPt alloy magnetic layers to crystalline lattice spacings of (110) faces in the top underlayer, i.e., the Cr alloy (Cr added with dissimilar metals) underlayer. That is, by minimizing differences between the crystalline lattice spacings of (002) faces in the CoPt alloy magnetic layers and the crystalline lattice spacing of (110) faces in the top underlayer, i.e., the Cr alloy underlayer, medium noise can be reduced while magnetic coercive force and overwriting properties are simultaneously improved.
Incidentally, it is not desirable that there is no difference between the crystalline lattice spacings of (002) faces in the CoPt alloy magnetic layers and the crystalline lattice spacings of (110) faces in the Cr alloy underlayers or in the Cr alloy spacer layers. A plenty of experiments revealed that some difference is preferred to reduce the noise. That is, medium noise is reduced by controlling the C-axis orientation of magnetic layers within a certain range.